Not common because they are expensive and the long overhangs of most C motorhomes don't work well with typical 4-wheeling environments. Tiger (http://www.tigervehicles.com/) makes the least expensive off-road motorhomes, building short houses on 4x4 pickup chassis. Xplorer once did, long enough ago that used examples might not be too expensive. Beyond that, you would be getting into excursion vehicles that cost hundreds of thousand of dollars new, whether built for recreation or industrial use.
If just for driving onto a beach, you can get a 4x4 conversion (http://www.quigley4x4.com/ for example) or maybe find one used. That can be on a camping van or cutaway with a larger Class C house. Which one works depends on how sharp the grads are getting to your beaches, e.g. the overhang on my 29-foot C hits a lot on short driveway ramps and some dips in city streets, on a beach that might just dig into to the sand.